lavf/img2enc: extend current options documentation

doc/muxers.texi

@@ -241,8 +241,13 @@ ffmpeg -i in.avi -f image2 -frames:v 1 img.jpeg
 @end example
 
 @table @option
-@item -start_number @var{number}
+@item start_number @var{number}
-Start the sequence from @var{number}.
+Start the sequence from @var{number}. Default value is 1. Must be a
+positive number.
+
+@item updatefirst 1|0
+If set to 1, update the first written image file again and
+again. Default value is 0.
 @end table
 
 The image muxer supports the .Y.U.V image file format. This format is

libavformat/img2enc.c

@@ -128,8 +128,8 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
 #define OFFSET(x) offsetof(VideoMuxData, x)
 #define ENC AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption muxoptions[] = {
-    { "updatefirst",  "",                            OFFSET(updatefirst), AV_OPT_TYPE_INT, { .i64 = 0 }, 0,       1, ENC },
+    { "updatefirst",  "update the first image file",      OFFSET(updatefirst), AV_OPT_TYPE_INT, { .i64 = 0 }, 0,       1, ENC },
-    { "start_number", "first number in the sequence", OFFSET(img_number), AV_OPT_TYPE_INT, { .i64 = 1 }, 1, INT_MAX, ENC },
+    { "start_number", "set first number in the sequence", OFFSET(img_number), AV_OPT_TYPE_INT,  { .i64 = 1 }, 1, INT_MAX, ENC },
     { NULL },
 };